Higher operating temperatures for gas turbines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the turbine must correspondingly increase. Significant advances in high-temperature capabilities have been achieved through the formulation of nickel and cobalt-based superalloys, though without a protective coating components formed from superalloys typically cannot withstand long service exposures if located in certain sections of a gas turbine, such as the turbine or combustor. One such type of coating is referred to as an environmental coating, i.e., a coating that is resistant to oxidation and hot corrosion. Environmental coatings that have found wide use include diffusion aluminide coatings formed by diffusion processes, such as a pack cementation, vapor phase processes and slurry processes.
Though significant advances have been made with environmental coating materials and processes for forming such coatings, there is the inevitable requirement to repair these coatings under certain circumstances. For example, removal may be necessitated by erosion or thermal degradation of the diffusion coating, refurbishment of the component on which the coating is formed, or an in-process repair of the diffusion coating or a thermal barrier coating (if present) adhered to the component by the diffusion coating. Known repair processes completely remove the diffusion aluminide coating by treatment with an acidic solution capable of interacting with and removing both the additive and diffusion coatings.
Removal of the entire aluminide coating, which includes the diffusion zone, results in the removal of a portion of the substrate surface. For gas turbine engine blade and vane airfoils, removing the diffusion zone can cause alloy depletion of the substrate surface and, for air-cooled components, excessively thinned walls and drastically altered airflow characteristics to the extent that the component must be scrapped. Therefore, rejuvenation processes have been developed for situations in which a diffusion aluminide coating must be refurbished in its entirety, but removal of the coating is not desired or allowed because of the effect on component life. Known rejuvenation processes, as shown in FIG. 1, generally include a deposition of an aluminum-infused additive layer 107 on the metallic substrate 101 along a substrate surface 103. When the component is in need of rejuvenation, such as after operation, the diffusion coating 105 including the aluminum-infused additive layer 107 and an interdiffusion zone 109 generally below the substrate surface 103 are fully removed, leaving a post-treatment surface 111 below the original exposed surface 103, resulting in lost wall thickness 113. The reduced wall thickness 113 results in a degradation of the component and reduced life cycles. This known aluminide refurbishment process undesirably removes about 0.7 mil thick wall of base materials or more while stripping the diffusion coating including interdiffusion zone 109.
From the above, it can be appreciated that improved methods for refurbishing a diffusion aluminide coating are desired. A method that can refurbish a coated article by forming diffusion aluminide coatings on metallic substrates that does not suffer from one or more of the above drawbacks would be desirable in the art.